1. Field of the Invention
The present invention relates to an image forming apparatus utilizing an electrophotographic system.
2. Description of the Related Art
An image forming apparatus utilizing an electrophotographic system, such as a copy machine, a printer or a fax machine, mainly consists of a photosensitive drum with a drum surface on which a toner image is formed, a developing device with a developing roller that supplies toner to the drum surface of the photosensitive drum to form the toner image on the drum surface, and a transfer roller that forms a nip part between the transfer roller and the drum surface and transfers the toner image on the drum surface to a sheet.
In this type of image forming apparatus, a developing bias is applied to the developing roller and a drum bias to the drum surface at the time of image formation, and consequently a potential difference is generated between the developing roller and the drum surface. This potential difference allows the toner to move from a circumferential surface of the developing roller onto the drum surface, forming the toner image on the drum surface. Also, application of a transfer bias to the transfer roller generates a potential difference between the drum surface and the transfer roller. Due to this potential difference, the toner image on the drum surface is attracted to the transfer roller and transferred onto the sheet passing through the nip part.
In the image forming apparatus with the configuration described above, the transfer of the toner image depends on a rise of the transfer bias. A rise of the transfer bias fluctuates according to various factors, such as, for example, the rising performance of the transfer bias in a high-voltage power source, as well as a hygrothermal environment. When the transfer bias rises slowly, a leading end part of the sheet is not applied with a sufficient level of transfer bias when passing through the nip part. As a result, the toner image is not transferred adequately to the leading end part of the sheet, causing an image defect in the leading end part of the sheet.
In a technology for inhibiting an image defect that is used in a conventional image forming apparatus, a low level of first transfer bias current is applied once to a leading end part of a sheet passing through a nip part, and then a second transfer bias current higher than the first transfer bias current is applied to a downstream side section of the sheet when the downstream side section from the leading end part passes through the nip part. Accordingly, transfer bias rising time can be shortened, inhibiting the generation of an image defect.
However, in the image forming apparatus using the conventional technology described above, although low, the first transfer bias current is applied to the leading end part of the sheet, to charge the leading end part of the sheet. Because the drum surface of the photosensitive drum is applied with a drum bias current having a polarity opposite to that of the first transfer bias current, the leading end part of the sheet is electrostatically attached to the drum surface. As a result, the separability of the sheet from the drum surface drops, causing a paper jam.